Caithness Field club Bulletin
April 1975

THE DISTRIBUTION OF FRESHWATER FISH IN THE BRITISH
ISLES WITH PARTICULAR REFERENCE TO CAITHNESS
D.0liver

* anadromous - salt water living, returning to fresh water to spawn and spending its early life there.

On first coming to Caithness one's impression of the Wick and Thurso rivers is that of slow moving canal-like bodies of water completely alien to Scotland and something akin to the rivers of the south of England. One would expect these rivers to contain coarse fish, such as roach, rudd, dace, chub and perch, but they contain few species, dominated by salmon and trout. This brief article is aimed at explaining this apparent peculiarity.

There are several factors governing the distribution of fish in the British Isles and the start of this distribution is the ice age.

As the ice sheet moved southwards all forms of life were obliterated sothat when the ice started to retreat a complete recolonisation of land and freshwater environments had to take place. The first of the vertebrates to recolonise was the char - an anadromous species*. As the climate improved the char followed the retreating ice (it is still anadromous in Greenland, Norway, and Iceland) but left behind pockets of purely freshwater living fish in the larger rivers and lochs.

Before arctic conditions passed away the lochs were colonised in a similar fashion by the white fish - Coregonus species such as the powan,, pollan, vendace, and schelly now only present in a few lochs to the west of the British Isles. Salmon and sea trout followed the coregonid fish - both species being anadromous, but only the sea trout forming many totally fresh-water colonies, (now called brown trout)in both still and running water almost completely replacing the freshwater colonies of char. These colonies of char remained only in large cold deep lochs (nearest to Arctic conditions) but this will be discussed later. The salmon did manage to form freshwater colonies outside of the British Isles in river systems where there were very large lochs - presumably large enough to support a large population of fish for example, L.Ladoga in USSR/Finland, L.Wener in Sweden and also lochs in Norway and the Eastern seaboard of Canada.

The advance and retreat of the ice sheets also had a large effect on the sea levels around Britain and Europe. Following the retreat of the ice a great weight was lifted off the land which rose, and the sea level fell. At this time the rivers of the south coast of England were tributaries of the rivers of France, and the east Coast rivers as far north as possibly Durham were tributaries of the Rhine. These rivers, of course, had trout, salmon, and char in them, but as the climate improved, conditions became suitable and coarse fish such as the minnow, dace, chub and grayling moved in from the continental rivers and ousted the trout-like fishes. These in turn were replaced by carp, tench and bream where conditions were suitable.

Following the ice retreat and raising of the land, large quantities water were released and this, in turn, led to the see level being raised. The southern North sea and the English channel came into existence, and so the spread of fish from Continental sources was cut off. Thus the ice age was responsible for the large number of species of fish in the south and east with a gradual diminution of species as one goes north and westwards. However, the coarse fish did arrive and colonise west flowing rivers by slower methods such as river capture, being carried on birds legs (several coarse fish produce eggs in long "strings" or with adhesive for sticking to vegetation) and of course, latterly, by man.

The second great influence on the distribution of freshwater fish in the British Isles is that of the right ecological conditions. For instance it would be impossible for a pike, which lies hidden in weeds waiting for its prey to pass, to live in a Highland torrent.

The groups of ecological conditions of rivers and the fish they support are summarised in the diagram. The geographical zones (with which readers will probably be most familiar) and botanical zones of the river are given alongside for comparison.

It must be noted that the proportion of these regions can differ greatly from those shown here, or may be absent altogether. For example, the river Wick has no headstream or Bream zone; the Aberdeenshire Dee has a long head-stream, shorter troutbeck and a very long minnow reach with no B1 or B2.

From the diagrams one can see the varying conditions needed to support different fish populations. In Caithness the Thurso and Wick rivers have all the conditions for a Barbel zone except the high summer temperature and hence low oxygen content so that salmon and trout can survive. Even if coarse fish of the Barbel zone arrived in the district through the various distribution methods mentioned above, it is doubtful if they would survive because these fish spawn in the summer and the young fish would not be able to grow fast enough on the cool water to put on enough weight to survive the rigours of the winter.

The south of England rivers at the end of the ice age would be in the headstream zone and as the ice retreated they would be colonised by the high oxygen low temperature species like char and trout. As conditions ameliorated these fish would be replaced firstly by fish of the A3 zone and then B1 zone and finally, if conditions and distributive factors allowed, by fish of the B3 zone.

The foregoing article has so far dealt with running water. Lakes or lochs also provide a suitable habitat for various species of fish, some of which like the char and coregonids are only found in lochs.

Lakes can be categorised into two min groups, Eutrophic and Oligotrophic.

The Eutrophic lake, as most lowland and continental lakes are, contain a lot of minerals for vegetative growth which is profuse; they are shallow and warm in summer. These lakes can support a varied population of fish, usually those of the Barbel and Bream zones.

The Oligotrophic lake is deeper and is characterised by the establishment of a thermocline in summer. This requires an explanation - in the summer only the surface waters heat up, i.e. down to a depth of 10 metres or so. At this depth the temperature drops, from its summer maximum of 20deg.C, very quickly over a few metres to a temperature around 4deg.C. In other words the less dense warmer water floats on the denser cold water. This temperature of 4deg.C is more or less constant all the year round. The oligotrophic lake has little vegetation, lower summer temperatures and contains fish of the A2 and A3 zone, e.g. trout, salmon, minnow, stoneloach and bullhead in the littoral zone. Pike and perch also inhabit oligotrophic lakes as well as the coregonids and char. These latter fish only live where they can move below the thermocline to cold water in summer. No doubt these fish when they first colonised the British waters were present in all types of water, rivers and lochs, shallow and deeper, but as summer temperatures increased they, being very susceptible to high temperatures retreated to the cold deep lochs and were eliminated from all other waters. Trout and salmon being slightly more tolerant of higher temperatures (and hence slightly lower oxygen content) have survived in these waters once colonised by the white fish and char; but they in turn have been replaced by other fish in eutrophic lakes the B1 and B2 zones of rivers.

Finally the third influence in the distribution of freshwater fishes in the British Isles is that of man. His influence started in the middle ages and the establishment of stew ponds in the monasteries. These fish, mainly carp, would escape from time to time and become established in neighbouring rivers. The Victorians spread a lot of trout around their sporting estates. New areas of rivers were opened up to spawning salmon by building salmon ladders. New species were introduced like the Rainbow trout from the Rocky mountains of both Canada and the southern United States. Also the American brook trout from the eastern part of the USA and Canada. (This is a char and not a trout but has adapted to living in warmer flowing water). Fishermen have been instrumental in spreading pike because they want to catch them as they grow to large specimen sizes. Salmon fishermen have introduced the minnow to as far north as Inverness from the river Forth area - they bring them north live to use as bait for salmon fishing - at the end of the salmon fishing holiday the surplus being released.

All-polluting man has been responsible for the present-day distribution of fish in that he has removed many species from large areas of the country, for instance many miles of rivers in Lancashire and Yorkshire are completely devoid of all species. The salmon has been completely eliminated from many rivers, e.g. Thames, Clyde, Yorkshire Ouse.

To summarise: the distribution of freshwater fish in the British Isles was governed by the recolonisation of rivers from European rivers as the ice retreated following the last ice age. When the sea level rose this form of recolonisation was removed. Certain fish are adapted to living in certain conditions - if the right conditions do not exist then the fish will not thrive there and finally man has been influential in spreading or removing fish from various parts of Britain.

Finally, the species list for Caithness is very small because of its northerly position - its rivers were never joined to those of the continent to allow fish to spread from there. It is too far north to have hot enough summers for the water to heat up sufficiently to allow coarse fish to thrive - the pike and perch would do so if introduced.

The Caithness species list includes salmon, trout (including the sea trout) three spined stickleback, flounder, eel, char (Loch Calder) and at least one species of lamprey - total of seven or eight species compared with over fifty British species.